A highly efficient electron transport layer (ETL) is an essential constituent for good performance and stability in planar perovskite solar cells. Among n-type metal oxide materials, zinc oxide (ZnO) is a promising candidate for an electron transport layer due to its relatively high electron mobility, high transparency, and versatile nanostructures. However, it was found that several disadvantages could occur at the ZnO/perovskite interface, such as decomposition of CH3NH3PbI3 and poorly aligned energy levels. To overcome these issues, we present a design based on staircase band alignment of a low-temperature solution-processed ZnO/Al-doped ZnO (AZO) bilayer thin film as electron transport layers in planar perovskite solar cells. Experimental results revealed that the power conversion efficiency (PCE) of perovskite solar cells was significantly increased from 12.3% to 16.1% by employing the AZO thin film as the buffer layer. Meanwhile, the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) were improved to 20.6 mA/cm2, 1.09 V, and 71.6%, respectively. The enhancement in performance is attributed to the modified interface in the ETL with staircase band alignment of ZnO/AZO/CH3NH3PbI3, which allows more efficient extraction of photogenerated electrons in the CH3NH3PbI3 active layer. Our studies demonstrated that the solution-processed ZnO/AZO bilayer ETLs provide a promising new approach for the development of low-cost, high-performance, and stable planar perovskite solar cells.
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